The present invention relates to elemental analyzers and particularly an analyzer employing one or more bidirectional ballasts for the collection of analytes. The new ballast system extends the range and performance capabilities of existing ballast-type CHN(S) organic instruments.
The determination of elements, such as carbon, hydrogen, and nitrogen, in an organic material is desirable for numerous reasons. In recent years, the food market has become interested in determining the amount of protein in a sample which can be determined by the nitrogen content. Thus, the determination of nitrogen is important in providing useful information to the nutritional market. The carbon-to-hydrogen ratio is desirable in the characterization of coal and coke samples, as are the carbon, hydrogen, and nitrogen ratios in a variety of other organic materials. Thus, elemental analyzers have been in use for these and other applications for some time.
In present organic combustion analyzers, such as a TruSpec® analyzer from Leco Corporation of St. Joseph, Mich., oxygen (O2) gas is passed through a high temperature furnace. The sample material is positioned in the furnace for combustion and subsequent analysis. The combustion gases are captured in movable piston ballast, typically 6 liters in volume and the gasses are pressurized to approximately 1.5 atm. After equilibration in the ballast, the gasses are exhausted through a 10 cc aliquot loop which is subsequently equilibrated at atmospheric pressure. This process results in approximately a 1/900 portion of the combustion gases sent on for further analysis. The balance of the collected gasses is exhausted without being analyzed. This system is described in U.S. Pat. No. 7,070,738, assigned to the present assignee, the disclosure of which is incorporated herein by reference. U.S. Pat. Nos. 7,497,991; 4,622,009; 6,291,802; and 6,270,727 also disclose components of a combustion system which can be employed in the system of the present invention. The disclosures of the '991, '009, '802, and '727 patents are also incorporated herein by reference.
In these elemental analyzers, the contents of the aliquot loop are transferred into a helium stream where water (H2O) and carbon dioxide (CO2) are measured using non-dispersive IR detector (NDIR) cells. Finally nitrogen (N2) is measured using a thermal conductivity (TC) cell. Because the TC cell is a non-discriminant detector, all other gases from combustion must first be scrubbed before measuring N2. A small aliquot loop is desirable because, if large doses were used, the scrubbers would have to be replaced much more often incurring cost and downtime.
The prior art leaves room for improvement. Initially, the ballast is sized for the upper range of the sample size and concentration. As a result, very small or low concentration samples may be over-diluted with O2 limiting the low end of the detected dynamic range of samples. The constant ballast volume also limits the high end because, once the ballast is filled, no additional gas can be collected. Also, the recovery of substances that combust more slowly (i.e., longer than the time to fill the ballast) may not occur. The analysis time is also fixed, based upon purge time of the furnace; the fill, equilibrate, and exhaust times of the large ballast; and upon the fill, equilibrate, and scrubbing times of the aliquot dose.